What constitutes a 2-step tst?

A 2 step TST is. Serial tests 2 weeks apart. This short interval implies an old positive test and not a skin test conversion.
The CDC knows! Here is a great page from the cdc that has all the information you need: http://www.Cdc.Gov/tb/publications/ltbi/diagnosis.Htm.
PPD. 2 step tst is bascially having a second PPD test repeated within 1 month of the first ppd.